I'm not entirely convinced Grubbin evolves into Charjabug. For starters they don't look really anything alike. People keep saying Charjabug is a cocoon but it looks way more like a grub.

There was no hint in the trailer that Grubbin evolved into Charjabug. Usually when they introduce an evolution to a previously announced Pokemon they show it in the trailer.

The only thing that seems to connect them are the pincers and the electricity thing. But there could very well be more than one bug/electric type this gen.